Heretofore, a fuel cell system including a fuel cell for receiving the supply of a reactant gas (a fuel gas and an oxidizing gas) to generate a power has been suggested and put to practical use. When the power is generated, impurities such as nitrogen and carbon monoxide are accumulated in the fuel cell of the fuel cell system and a circulation passage of a fuel off gas with an elapse of time. To discharge such impurities from the system, a technology (a purge technology) is suggested in which a gas discharge valve is provided in a discharge flow passage connected to the circulation passage, and the opening/closing of the gas discharge valve is performed to discharge the gas from the circulation passage after each predetermined time.
Moreover, at present, there have been suggested a technology (Japanese Patent Application Laid-Open No. 2004-179000) for performing control so as to close the gas discharge valve in a case where the flow rate of the gas flowing through the gas discharge valve exceeds a predetermined value and a technology (Japanese Patent Application Laid-Open No. 2005-141977) for setting discharge time in accordance with the power generation state of the fuel cell to realize the discharge of the gas of a discharge amount equal to a required discharge amount corresponding to the power generation state.